1. Field of the Invention
The present invention relates generally to protective circuits for electronic devices, and more specifically to overvoltage protection circuits suitable for use in protecting devices attached to telephone lines.
2. Description of the Prior Art
In order to prevent damage from electrical overvoltages, devices attached to electrical lines such as telephone lines are protected by overvoltage protection circuitry designed to accomplish this task. Such overvoltages can be caused by, for example, lightning strikes somewhere in the system, or power surges caused by accidental cross connection with power lines. The overvoltage protection circuitry should protect the primary devices, and reset to allow normal operation after the overvoltage condition has passed.
In the past, numerous different circuit designs have been used to provide protection. Typically, such designs utilize 2 or three discrete semiconductor devices packaged into a single component package. These devices can provide good protection, but the use of more than one semiconductor die in a package results in a relatively expensive protection circuit.
Overvoltage protection circuitry intended for use with telephone equipment must take into account the particular hazards which are encountered when devices are connected to the telephone system. Two incoming signal lines, called "tip" and "ring" for historical reasons, carry the normal telephone signal. Overvoltages can occur between these two lines. More commonly, overvoltages can occur between one or both of these lines and ground. In normal operation, the voltage on the tip and ring lines float with respect to ground, although one line is typically at approximately -2 volts, and the other at approximately -50 volts. A 48 volt differential between the two lines is expected in normal operation.
In order to provide the most complete protection, overvoltage potentials must be protected against between the tip and ring lines as well as between these lines and ground. Thus, in most applications a three-way balanced protective circuit is preferred, although two-way balanced circuits are useful and common as known in the art. Examples of devices which have been utilized in the past to provide the required protection can be found in U.S. Pat. No. 4,282,555, titled OVERVOLTAGE PROTECTION MEANS FOR PROTECTING LOW POWER SEMICONDUCTOR COMPONENTS, and U.S. Pat. No. 4,905,119, titled SOLID STATE OVERVOLTAGE PROTECTION CIRCUIT.
Although the circuits described in these patents can provide adequate protection for devices attached to telephone lines, they are difficult to produce inexpensively in a monolithic integrated circuit. The circuit in U.S. Pat. No. 4,905,119 utilizes more than one chip, which increases cost of the overall circuit. The device described in the U.S. Pat. No. 4,282,555 patent can be implemented on a single chip, but P-type diffusions through the depth of the device must be used for isolation. This process adds greatly to the cost of the device.
An object of the invention is to provide a design for an overvoltage protection circuit which can be easily and inexpensively integrated into a single monolithic device.
Another object of the invention is to provide such a circuit that is suitably turned off after an overvoltage in use of a connection with a telephone line.
Another object of the invention is to provide such a circuit that is triggered not only further to an overvoltage, but also further to an overcurrent.
Another object of the invention is to provide such a design which is flexible enough to be easily changed to provide several different protection techniques.
Another object of the invention is to provide such a design to incorporating packaging designs which are flexible and easily adapted to a number of different mounting and thermal dissipation requirements.